This invention relates generally to the casting of metal sheets and is particularly directed to the horizontal electromagnetic casting of thin metal sheets.
Steel making occupies a central economic role and represents a significant fraction of the energy consumption of many industrialized nations. The bulk of steel making operations involves the production of steel plate and sheet.
Present steel mill practice typically produces thin steel sheets by pouring liquid steel into a mold, whereupon the liquid steel solidifies upon contact with the cold mold surface. The solidified steel leaves the mold either as an ingot or as a continuous slab after it is coooled typically by water circulating within the mold wall during a solidification process. In either case, the solid steel is relatively thick, e.g., 6 inches or greater, and must be subsequently processed to reduce the thickness to the desired value and to improve metallurgical properties. The mold-formed steel is usually characterized by a surface roughened by defects, such as cold folds, liquations, hot tears and the like which result primarily from contact between the mold and the solidifying metallic shell. In addition, the steel ingot or sheet thus cast also frequently exhibits considerable alloy segregation in its surface zone due to the initial cooling of the molten surface from contact with the mold, reheating of the metal surface after mold contact, and then finally cooling of the metal surface from the direct application of a coolant. Subsequent fabrication steps, such as rolling, extruding, forging and the like, usually require the scalping of the ingot or sheet prior to working to remove both the surface defects as well as the alloy deficient zone adjacent to its surface. These additional steps, of course, increase the complexity and expense of steel production.
Steel sheet thickness reduction is accomplished by a rolling mill which is very captial intensive and consumes large amounts of energy. The rolling process therefore contributes substantially to the cost of the steel sheet. In a typical installation, a 10 inch thick steel slab must be manipulated by at least ten rolling machines to reduce its thickness. The rolling mill may extend as much as one-half mile and cost as much as $500 million.
Compared to currecnt practice, a large reduction in steel sheet total cost and in the energy required for its production could be achieved if the sheets could be cast in near net shape, i.e., in a shape and size closely approximating the final desired product. This would reduce the rolling mill operation and would result in a large savings in energy. There are several technologies currently under development which attempt to achieve these advantages by forming the thin steel sheets in the casting process. While some of the approaches under investigation use electromagnetic energy, all of these approaches use a solid mold on one or both sides of the sheet. One disadvantage of a mechanical mold is that contact between the molten metal and the solid mold wall often produces an undesirable surface finish which requires subsequent processing to correct as pointed out above.
The use of electromagnetic levitation techniques has been employed for some time in the aluminum industry. The practice there is to use electromagnetic fields to contain the top inch or so of a large, thick ingot. The molten aluminum is cooled and solidifies before it touches any mechanical support. Examples of this approach can be found in U.S. Pat. Nos. 3,467,166 to Getselev et al, 3,985,179 to Goodrich et al, 4,126,175 to Getselev, 4,161,206 to Yarwood et al, and 4,375,234 to Pryor.
Electromagnetic levitation of an electrically conducting molten body occurs when an alternating magnetic field generates induced eddy currents in the conducting material, primarily at its surface, and the induced currents interact with the external magnetic field to produce a magnetic pressure acting normal to the surface. If a sufficiently strong magnetic pressure is directed vertically upward, it can counteract the downward force of gravity on the body.
One of the difficulties with electromagnetic casting of metals involves the heating of the metal by eddy currents caused by the alternating magnetic field that levitates the molten metal. The heating by the magnetic field must be significantly less than the energy that can be removed by the cooling system or the molten metal will not solidify quickly. The heating increases as the static pressure head increases, and in the case of vertical levitation of a molten sheet, the pressure head may be several cm.
Horizontal electromagnetic casting offers several advantages over vertical casting. First, the entire casting system, including the post-solidification rollers can be on one floor of the factory, saving capital expense for the entire process. Second, the static pressure head that the magnetic fields must support need not be much more than the thickness of the plate to be cast. Thus, low field strength magnets can be used for the levitation.
A problem that is common to all electromagnetic levitation methods is the stability of the object to be levitated. The object must experience a restoring force whenever it departs from its intended equilibrium position. In the case of horizontal levitation, the upward net magnetic force must increase as the bottom surface of the molten sheet moves down, and the force must decrease as the bottom surface moves up. For the case of an isolated molten sheet suspended in a magnetic field, this requires that the horizontal component of the magnetic field decrease with height to obtain stability. To obey Maxwell's equations this requires that the vertical component of the magnetic field vary substantially across the sheet. The vertical component of the field contributes nothing to the levitation but contributes substantially to the eddy current heating. This heating in the horizontal case imposes a maximum on the width of the molten sheet.
In the horizontal levitation method, one way to avoid the large eddy heating of the vertical fields is to make the horizontal fields nearly constant. There is then no levitation in that the magnetic pressure on top of the molten sheet is the same as the magnetic pressure on the bottom, and no net magnetic force is generated on the molten sheet. The invention described herein overcomes this and other problems.